Aviation Investigation Report A98H0003

4.3.2  Aircraft System Evaluation: Fire-Hardening Considerations

In its material flammability standards recommendation package issued in August 2001, the TSB identified a deficiency regarding the certification of certain aircraft systems. In its recommendation A01-04, the TSB stated that more validation needed to be done prior to the certification of aircraft systems to ensure that a fire-induced material failure would not exacerbate the consequences of an in-flight fire. The response from the regulatory authorities supported the status quo by declaring that the regulations governing the certification of critical systems, such as hydraulic, oxygen, and flight controls were comprehensive enough to address a system's fire protection and prevention requirements. For other aircraft systems, regulatory authorities have indicated that the combined effect of increasing the material flammability standards, introducing new technologies like the AFCBs, and implementing the recommendations of the Wire Systems Harmonization Working Group will mitigate the risk of initiating or sustaining an in-flight fire.

Testing during the investigation demonstrated that the flight crew oxygen system in the MD-11 could fail in a high heat environment, and exacerbate a fire. The regulatory authorities have not addressed the issue of how the existing regulations allowed for the certification of this oxygen system, which was constructed using dissimilar metals, while providing for the "fire protection and prevention" certification requirement. The design of the oxygen system met the requirements of existing regulations, otherwise, it would not have been approved for use in an aircraft. The same holds true for other materials that failed and exacerbated the SR 111 fire, such as the silicon elastomeric end caps on the air conditioning ducts.

The Board disagrees that the eventual reduction or elimination of flammable materials, and anticipated technological advances, adequately deal with the near-term risk. Therefore, the Board is concerned that regulatory authorities have not taken sufficient action to mitigate the risks identified in the TSB's recommendation A01-04, issued in August 2001, which recommended that as a prerequisite to certification, all aircraft systems in the pressurized portion of an aircraft, including their subsystems, components, and connections, be evaluated to ensure that those systems whose failure could exacerbate a fire-in-progress are designed to mitigate the risk of fire-induced failures.

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